Power receiving device and wireless power supply system

ABSTRACT

Provided is a power receiving device in which supply of power from a power supply device can be stopped while a reduction in Q-value is suppressed. The power receiving device includes a first antenna which forms resonant coupling with an antenna of the power supply device; a second antenna which forms electromagnetic induction coupling with the first antenna; a rectifier circuit including a plurality of switches and performing a first operation or a second operation depending on whether the plurality of switches is ON or OFF, the first operation being an operation in which voltage applied from the second antenna is rectified to be outputted, and the second operation being an operation in which a pair of power supply points is short-circuited; a load to which the voltage is applied; and a control circuit which generates a signal for selecting ON or OFF of the plurality of switches.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power receiving device thatwirelessly receives power, a wireless power supply system including thepower receiving device, and a wireless power supply method.

2. Description of the Related Art

A wireless power supply technique for wirelessly supplying power from apower supply device to a power receiving device by electromagneticinduction has been developed and come into practical use. In recentyears, a wireless power supply technique for supplying power byelectromagnetic resonance (electromagnetic resonant coupling) thatenables long-distance power transmission as compared to a wireless powersupply technique for supplying power by electromagnetic induction hasattracted attention. Unlike by electromagnetic induction, byelectromagnetic resonance, high power transmission efficiency can bemaintained even when the transmission distance is several meters, andpower loss due to misalignment of an antenna of a power supply deviceand an antenna of a power receiving device can be reduced.

Patent Document 1 and Non-Patent Document 1 disclose wireless powersupply techniques utilizing electromagnetic resonance.

In electromagnetic resonant wireless power supply disclosed in PatentDocument 1 and Non-Patent Document 1, a power supply device and a powerreceiving device each include two antennas. Specifically, the powersupply device includes an antenna to which power is supplied from apower source through a contact and a resonant antenna that is coupledwith the antenna by electromagnetic induction. Further, the powerreceiving device includes a power receiving antenna for supplying powerto a load through a contact and a resonant antenna that is coupled withthe antenna by electromagnetic induction. When the resonant antenna ofthe power supply device and the resonant antenna of the power receivingdevice are coupled with each other by magnetic resonance or electricfield resonance, power is wirelessly supplied from the power supplydevice to the power receiving device.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2010-219838.-   [Non-Patent Document 1] Andre Kurs et al., “Wireless Power Transfer    via Strongly Coupled Magnetic Resonances”, Science, Jul. 6, 2007,    Vol. 317, pp. 83-86.

SUMMARY OF THE INVENTION

Further, in electromagnetic resonance, as described above, thetransmission distance can be set to be long and the allowable range ofpositional deviation between resonant antennas is wide. Therefore,restriction on a positional relation between a power supply device and apower receiving device is smaller in electromagnetic resonance than inelectromagnetic induction, which is advantageous when power is suppliedto a plurality of power receiving devices. However, in the case of aplurality of power receiving devices, even when the resonance frequencyof the power supply device is equal to that of the power receivingdevice, power transmission efficiency of the total power transmittedfrom the power supply device to the plurality of power receiving devicesis lower than in the case where one power supply device is paired withone power receiving device because resonant antennas of the plurality ofpower receiving devices interfere with each other. Further, in a powerreceiving device in which charging is completed and supply of power isunnecessary, a circuit element, a wiring, and the like that areconnected to a power receiving antenna are charged and discharged, whichcauses power consumption. In addition, supply of power from the powersupply device to the power receiving device in which charging iscompleted is not stopped, whereby transmission efficiency of powersupplied to a power receiving device while charging remains low.

One of effective methods to increase power transmission efficiency is toshort-circuit a pair of power supply points of a coil of a resonantantenna in the power receiving device in which charging is completed ofthe plurality of power receiving devices by a switch. When the pair ofpower supply points of the coil is short-circuited, magnetic resonantcoupling or electric field resonant coupling formed between a resonantantenna of the power supply device and the resonant antenna of the powerreceiving device can be decoupled. Accordingly, the power receivingdevice in which charging is completed hardly inhibits magnetic resonancecoupling or electric field resonance coupling between the power supplydevice and another power receiving device, which leads to an increase inpower transmission efficiency.

However, in this method, it is necessary to provide, in the resonantantenna, a switch for a short circuit and a wiring, a circuit element,and the like for selecting ON or OFF of the switch, which causes anincrease in resistance of the whole resonant antenna. Therefore, itcannot be said that the above method is preferable because inelectromagnetic resonance, Q-value is reduced due to an increase inresistance of the whole resonant antenna to decrease power transmissionefficiency.

In view of the foregoing technical background, an object of oneembodiment of the present invention is to provide a power receivingdevice in which supply of power from a power supply device can bestopped while a reduction in Q-value is suppressed. Further, an objectof the present invention is to propose a wireless power supply systemwith high efficiency of power transmission or a wireless power supplymethod with high efficiency of power transmission, in which the powerreceiving device is used.

In one embodiment of the present invention, a pair of power supplypoints of an antenna element of a power receiving antenna in a powerreceiving device is short-circuited using a rectifier circuit of thepower receiving device. Specifically, the rectifier circuit includes oneor more switches for electrically connecting the pair of power supplypoints to each other. Any of the one or more switches is ON, so that thepair of power supply points of the antenna element can beshort-circuited. Further, in the rectifier circuit, any of the one ormore switches is ON or OFF in accordance with AC voltage generated by anAC power source of the power supply device, so that the potential of oneof the power supply points of the antenna element is outputted from therectifier circuit.

More specifically, a power receiving device according to one embodimentof the present invention includes a first antenna which forms magneticresonance coupling or electric field resonance (hereinafter simplyreferred to as resonance) coupling with an antenna of the power supplydevice; a second antenna which forms electromagnetic induction couplingwith the first antenna; a rectifier circuit including a plurality ofswitches and performing a first operation or a second operationdepending on whether each of the plurality of switches is ON or OFF, thefirst operation being an operation in which voltage applied from thesecond antenna is rectified to be outputted, and the second operationbeing an operation in which a pair of power supply points of an antennaelement of the second antenna is short-circuited; a load to which thevoltage outputted from the rectifier circuit is applied; and a controlcircuit which generates a signal for selecting ON or OFF of each of theplurality of switches in the first operation and the second operation bythe rectifier circuit.

Further, the power receiving device according to one embodiment of thepresent invention may include a receiving circuit which wirelesslyreceives a signal including, as data, a cycle of the AC voltagegenerated in the power supply device. The receiving circuit includes anantenna, a rectifier circuit, a demodulation circuit, and the like. Withthe use of the signal including the cycle of the AC voltage as data, thecontrol circuit generates a signal for selecting ON or OFF of each ofthe plurality of switches in the rectifier circuit in the firstoperation in accordance with the cycle of the AC voltage generated inthe power supply device.

Further, the power receiving device according to one embodiment of thepresent invention may include a power storage device such as a secondarybattery or a capacitor as the load. The control circuit generates asignal for selecting ON or OFF of each of the plurality of switches sothat the rectifier circuit performs the first operation while chargingof the power storage device is performed. Further, the control circuitgenerates a signal for selecting ON or OFF of each of the plurality ofswitches so that the rectifier circuit performs the second operationwhile charging of the power storage device is not performed.

In the power receiving device according to one embodiment of the presentinvention, the pair of power supply points of the antenna element of thepower receiving antenna is short-circuited. With the above structure,supply of power to a circuit element, a wiring, and the like that areconnected to the power receiving antenna is stopped; therefore, theresonant antenna of the power receiving device does not substantiallyreceive power from the resonant antenna of the power supply device.Therefore, in the power receiving device according to one embodiment ofthe present invention, supply of power from the power supply device canbe stopped without a short-circuit between the pair of power supplypoints of the antenna element of the resonant antenna. Further, in awireless power supply system or a wireless power supply method accordingto one embodiment of the present invention, the stop of the supply ofpower from the power supply device to the power receiving device inwhich charging is completed can improve power transmission efficiencyfrom the power supply device to another power receiving device.

In one embodiment of the present invention, with the above structure, apower receiving device in which supply of power from a power supplydevice can be stopped while a reduction in Q-value is suppressed can beprovided. Further, in one embodiment of the present invention, awireless power supply system with high efficiency of power transmissionor a wireless power supply method with high efficiency of powertransmission, in which the power receiving device is used, can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a structure of a wireless power supply system.

FIG. 2 illustrates a structure of a wireless power supply system.

FIGS. 3A and 3B each show a timing chart.

FIGS. 4A and 4B show conceptual diagrams each illustrating operation ofa wireless power supply system.

FIG. 5 illustrates a structure of a power receiving device.

FIGS. 6A to 6C each illustrate conditions of an experiment.

FIG. 7 illustrates a relation between loss of power transfer andfrequency f.

FIG. 8 illustrates a structure of a power receiving device.

FIG. 9 illustrates a structure of a power supply device.

FIGS. 10A and 10B each show a timing chart.

FIGS. 11A and 11B each illustrate a state of wireless power supply.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description and it is easilyunderstood by those skilled in the art that the mode and details can bevariously changed without departing from the scope and spirit of thepresent invention. Therefore, the present invention should not beinterpreted as being limited to the description in the followingembodiments.

Embodiment 1

FIG. 1 illustrates an example of a wireless power supply systemaccording to one embodiment of the present invention. The wireless powersupply system in FIG. 1 includes a power receiving device 100 and apower supply device 200.

The power receiving device 100 includes a resonant antenna 101, a powerreceiving antenna 102, a rectifier circuit 103, a control circuit 104, aload 105, and a receiving circuit 106. Further, the power supply device200 includes a resonant antenna 201, an antenna 202, an AC power source203, a control circuit 204, and a transmitting circuit 205.

First, a specific structure of the power supply device 200 is described.

The resonant antenna 201 includes an antenna element 206 that is aninductor and capacitance in the antenna element 206. Further, in orderto adjust the resonant frequency of the resonant antenna 201, acapacitor may be connected to the antenna element 206 in addition to thecapacitance in the antenna element 206. In FIG. 1, the capacitance inthe antenna element 206 and the capacitor for adjusting the resonantfrequency are collectively referred to as a capacitor 207. The resonantantenna 201 in FIG. 1 is shown in an equivalent circuit in which theantenna element 206 and the capacitor 207 are connected to each other.

The antenna element 206 can be a spiral conductor, a loop conductor, ahelical conductor, or the like. The inductance of the antenna element206 and the capacitance of the capacitor 207 are set so that theresonant frequency of the resonant antenna 201 is equal to the resonantfrequency of the resonant antenna 101 of the power receiving device 100.With the above structure, resonant coupling can be formed between theresonant antenna 201 and the resonant antenna 101. Note that resonantcoupling means a state in which power or a signal is wirelesslytransmitted and received by resonance.

The antenna 202 includes an antenna element 208 that is an inductor. Acapacitance may exist in the antenna element 208 or a capacitor may beconnected to the antenna element 208. In FIG. 1, the capacitance in theantenna element 208 and the capacitor for adjusting the resonantfrequency are collectively referred to as a capacitor 209. The antenna202 in FIG. 1 is shown in an equivalent circuit in which the antennaelement 208 and the capacitor 209 are connected to each other.

Further, as in the antenna element 206, the antenna element 208 can be aspiral conductor, a loop conductor, a helical conductor, or the like.Note that in the antenna 202, the shape (e.g., diameter) of the antennaelement 208 and the positional relationship between the antenna element206 and the antenna element 208 are set so that the magnitude ofmagnetic flux that is outputted from the antenna 202, is interlinkedwith the resonant antenna 201, and contributes to induced electromotiveforce in the resonant antenna 201, that is, the magnitude of mainmagnetic flux increases. Specifically, it is preferable that thediameter of the antenna element 208 be larger than a distance betweenthe antenna element 206 and the antenna element 208 in order to improvepower transmission efficiency between the resonant antenna 201 and theantenna 202. With the above structure, electromagnetic inductioncoupling can be formed between the antenna 202 and the resonant antenna201. Note that electromagnetic induction coupling means a state in whichpower or a signal is wirelessly transmitted and received byelectromagnetic induction.

The AC power source 203 has a function of supplying AC voltage to theantenna 202. A cycle of the AC voltage which is supplied from the ACpower source 203 to the antenna 202 is controlled by the control circuit204. The transmitting circuit 205 has a function of wirelesslytransmitting a signal including the cycle as data to the power receivingdevice 100 when the signal is supplied from the control circuit 204.Specifically, the transmitting circuit 205 includes a modulation circuitor the like and, by applying modulation to AC voltage applied to theantenna 202, superimposes the signal including the cycle as data on aradio wave transmitted from the antenna 202.

Next, a specific structure of the power receiving device 100 isdescribed.

The resonant antenna 101 includes an antenna element 107 that is aninductor and capacitance in the antenna element 107. Further, in orderto adjust the resonant frequency of the resonant antenna 101, acapacitor may be connected to the antenna element 107 in addition to thecapacitance in the antenna element 107. In FIG. 1, the capacitance inthe antenna element 107 and the capacitor for adjusting the resonantfrequency are collectively referred to as a capacitor 108. The resonantantenna 101 in FIG. 1 is shown in an equivalent circuit in which theantenna element 107 and the capacitor 108 are connected to each other.

The antenna element 107 can be a spiral conductor, a loop conductor, ahelical conductor, or the like. The inductance of the antenna element107 and the capacitance of the capacitor 108 are set so that theresonant frequency of the resonant antenna 101 is equal to the resonantfrequency of the resonant antenna 201 of the power supply device 200.With the above structure, resonant coupling can be formed between theresonant antenna 101 and the resonant antenna 201.

The power receiving antenna 102 includes an antenna element 109 that isan inductor. As in the antenna element 107, a capacitance may exist inthe antenna element 109 or a capacitor may be connected to the antennaelement 109. Further, as in the antenna element 107, the antenna element109 can be a spiral conductor, a loop conductor, a helical conductor, orthe like. Note that in the power receiving antenna 102, the shape (e.g.,diameter) of the antenna element 109 and the positional relationshipbetween the antenna element 107 and the antenna element 109 are set sothat the magnitude of magnetic flux that is outputted from the resonantantenna 101, is interlinked with the power receiving antenna 102, andcontributes to induced electromotive force in the power receivingantenna 102, that is, the magnitude of main magnetic flux increases.Specifically, it is preferable that the diameter of the antenna element109 be larger than a distance between the antenna element 107 and theantenna element 109 in order to improve power transmission efficiencybetween the resonant antenna 101 and the power receiving antenna 102.With the above structure, electromagnetic induction coupling can beformed between the power receiving antenna 102 and the resonant antenna101.

Power supply points A1 and A2 of the power receiving antenna 102 areconnected to input terminals B1 and B2 of the rectifier circuit 103,respectively. Accordingly, the potential of the power supply point A1 isapplied to the input terminal B1 and the potential of the power supplypoint A2 is applied to the input terminal B2.

Note that the term “connection” in this specification refers toelectrical connection through a contact and corresponds to the state inwhich current, a potential, or voltage can be supplied or transmittedthrough a contact. Accordingly, a connection state means not only astate of a direct connection but also a state of indirect connectionthrough a circuit element such as a wiring, a resistor, a diode, or atransistor so that current, a potential, or voltage can be supplied ortransmitted.

The rectifier circuit 103 includes a plurality of switches. FIG. 1illustrates a specific example in which the rectifier circuit 103includes switches 110 and 111. Further, the rectifier circuit 103includes a capacitor 112 in FIG. 1. Note that the rectifier circuit 103may further include another circuit element such as a transistor, adiode, a resistor, a capacitor, or an inductor as needed.

The switch 110 has a function of controlling connection between theinput terminal B1 and an output terminal C1 of the rectifier circuit103. That is, when the switch 110 is ON, the potential of the powersupply point A1 applied to the input terminal B1 is applied to theoutput terminal C1 through the switch 110. When the switch 110 is OFF,the potential of the power supply point A1 applied to the input terminalB1 is not applied to the output terminal C1.

Further, the switch 111 has a function of controlling connection betweenthe input terminal B2 and the output terminal C1 of the rectifiercircuit 103. That is, when the switch 111 is ON, the potential of thepower supply point A2 applied to the input terminal B2 is applied to theoutput terminal C1 through the switch 111. When the switch 111 is OFF,the potential of the power supply point A2 applied to the input terminalB2 is not applied to the output terminal C1 through the switch 111.

In one embodiment of the present invention, the rectifier circuit 103can perform two operations by selection of ON or OFF of each of theplurality of switches.

First, in a first operation, one of the switches 110 and 111 and theother thereof are alternately ON and OFF, repeatedly, whereby AC voltageapplied between the power supply points A1 and A2 is rectified.Switching of ON and OFF of the switches 110 and 111 is performed inaccordance with the cycle of the AC voltage applied between the powersupply points A1 and A2. DC voltage that can be obtained byrectification of the AC voltage is applied between the output terminalC1 and an output terminal C2.

One of electrodes of the capacitor 112 is connected to the outputterminal C1 and the other of the electrodes of the capacitor 112 isconnected to the output terminal C2. Further, a reference potential suchas a ground potential is applied to the output terminal C2 and apotential difference between the output terminals C1 and C2 is smoothedby the capacitor 112. Accordingly, the smoothed potential differencebetween the output terminals C1 and C2 is applied to the load 105 as ACvoltage.

Further, in a second operation, the switches 110 and 111 are ON, so thatthe power supply points A1 and A2 are short-circuited. When the powersupply points A1 and A2 are short-circuited, the potentials of the powersupply points A1 and A2 are substantially equal to the referencepotential applied to the output terminal C2. Therefore, in oneembodiment of the present invention, when the second operation isperformed in the rectifier circuit 103, supply of power to the rectifiercircuit 103, the load 105, another circuit element, another wiring, andthe like that are connected to the power receiving antenna 102 can bestopped. Accordingly, resonant coupling is not substantially formedbetween the resonant antenna 101 of the power receiving device 100 andthe resonant antenna 201 of the power supply device 200.

The plurality of switches of the rectifier circuit 103 is controlled inresponse to a signal for selecting ON or OFF that is transmitted fromthe control circuit 104. Therefore, when the plurality of switches ofthe rectifier circuit 103 is controlled by the control circuit 104,whether the first operation or the second operation is performed in therectifier circuit 103 is selected. Specifically, in the case where poweris wirelessly supplied from the power supply device 200 to the powerreceiving device 100, the first operation is performed in the rectifiercircuit 103 in response to a signal from the control circuit 104.Further, in the case where wireless power supply from the power supplydevice 200 to the power receiving device 100 is stopped, the secondoperation is performed in the rectifier circuit 103 in response to asignal from the control circuit 104.

Note that the signal for controlling the plurality of switches in thecontrol circuit 104 may be generated in response to a command inputtedfrom an input device or the like or a signal from the load 105. Notethat the input of a command from the input device may be performedmanually or performed in accordance with a distance between anotherelectronic device and the power receiving device 100 which is detectedby a mechanism provided in the input device.

The receiving circuit 106 receives the signal including the cycle of theAC voltage as data and transmitted from the transmitting circuit 205.Specifically, the receiving circuit 106 includes a demodulation circuitor the like and has a function of extracting the signal including, asdata, the cycle from AC voltage received by the power receiving antenna102.

Then, the signal received by the receiving circuit 106 is applied to thecontrol circuit 104. In the control circuit 104, when AC voltage isrectified in the first operation, the switching of ON and OFF of theswitches 110 and 111 is determined using the signal from the receivingcircuit 106.

Note that FIG. 1 illustrates an example in which a signal is transmittedand received between the transmitting circuit 205 and the receivingcircuit 106 through a group of antennas for supplying power, that is,the antenna 202, the resonant antenna 201, the resonant antenna 101, andthe power receiving antenna 102. However, in one embodiment of thepresent invention, a signal may be transmitted and received between thetransmitting circuit 205 and the receiving circuit 106 by a group ofantennas different from the group of antennas for supplying power.

FIG. 2 illustrates an example of a wireless power supply systemaccording to one embodiment of the present invention in the case where asignal is transmitted and received between the transmitting circuit 205and the receiving circuit 106 by a group of antennas different from thegroup of antennas for supplying power. The wireless power supply systemin FIG. 2 is different from the wireless power supply system in FIG. 1in that an antenna 210 connected to the transmitting circuit 205 and anantenna 113 connected to the receiving circuit 106 are additionallyprovided.

In FIG. 2, the transmitting circuit 205 includes at least an oscillatorcircuit in addition to the modulation circuit. In the transmittingcircuit 205, the modulation circuit modulates AC voltage outputted fromthe oscillator circuit, whereby the signal including the cycle of the ACvoltage as data is superimposed on a radio wave outputted from theantenna 210. When the antenna 113 receives the radio wave, AC voltagegenerated by reception of the radio wave is transmitted to the receivingcircuit 106. The receiving circuit 106 in FIG. 2 includes a demodulationcircuit or the like as in FIG. 1. The receiving circuit 106 has afunction of extracting the signal including, as data, the cycle from theAC voltage transmitted from the antenna 113.

Note that in FIG. 2, the antenna 210 and the antenna 113 may each havean antenna or a plurality of antennas.

Alternatively, modulation may be applied to a carrier wave between theresonant antenna 101 of the power receiving device 100 and the resonantantenna 201 of the power supply device 200 with the antenna 210, wherebya signal is transmitted from the transmitting circuit 205 to thereceiving circuit 106. In this case, since a signal is transmitted fromthe power receiving antenna 102 to the receiving circuit 106, theantenna 113 is not needed.

Further, in one embodiment of the present invention, the transmission ofa signal from the transmitting circuit 205 to the receiving circuit 106can be performed by a communication method in accordance with anexisting communication standard, for example, infrared communication, anear field communication method, or the like.

In the wireless power supply systems according to one embodiment of thepresent invention which are illustrated as examples in FIG. 1 and FIG.2, the power supply device 200 is provided with the antenna 202, wherebythe resonant antenna 201 is not in contact with the AC power source 203.With the above structure, in the power supply device 200, the resonantantenna 201 can be electrically isolated from the internal resistance ofthe AC power source 203. Further, the power receiving device 100 isprovided with the power receiving antenna 102, whereby the resonantantenna 101 is not in contact with the rectifier circuit 103 or the load105. With the above structure, in the power receiving device 100, theresonant antenna 101 can be electrically isolated from the internalresistance of the rectifier circuit 103 or the load 105. Thus, ascompared to the case where the resonant antenna 201 is connected to theAC power source 203 or the case where the resonant antenna 101 isconnected to the rectifier circuit 103 or the load 105, the Q-values ofthe resonant antenna 201 and the resonant antenna 101 are increased.Consequently, power transmission efficiency can be improved.

Next, the first operation and the second operation of the powerreceiving device 100 according to one embodiment of the presentinvention are specifically described using the wireless power supplysystem in FIG. 1 as an example.

First, when AC voltage is outputted from the AC power source 203 in thepower supply device 200, the power is wirelessly supplied to theresonant antenna 201 by electromagnetic induction coupling between theantenna 202 and the resonant antenna 201. Then, the power supplied tothe resonant antenna 201 is wirelessly supplied to the resonant antenna101 by resonant coupling between the resonant antenna 201 and theresonant antenna 101. Further, the power supplied to the resonantantenna 101 is supplied to the power receiving antenna 102 byelectromagnetic induction coupling between the resonant antenna 101 andthe power receiving antenna 102.

In the case where the first operation is performed in the rectifiercircuit 103 of the power receiving device 100, the switches 110 and 111operate in accordance with a timing chart in FIG. 3A. In FIG. 3A, apotential difference between the power supply points A1 and A2 when thepotential of the power supply point A2 is regarded as a referencepotential in the power receiving antenna 102 is shown as voltage V_(p).

According to the timing chart in FIG. 3A, when the voltage V_(p) islow-level voltage, that is, when the potential of the power supply pointA2 is higher than that of the power supply point A1, the switch 110 isOFF and the switch 111 is ON. Accordingly, the potential of the powersupply point A2, which is higher than that of the power supply point A1,is applied to the output terminal C1 through the switch 111. Further,according to the timing chart in FIG. 3A, when the voltage V_(p) ishigh-level voltage, that is, when the potential of the power supplypoint A1 is higher than that of the power supply point A2, the switch110 is ON and the switch 111 is OFF. Accordingly, the potential of thepower supply point A1 that is higher than that of the power supply pointA2 is applied to the output terminal C1 through the switch 110.

By the above first operation, a potential that is higher than thepotential of the output terminal C2 is applied to the output terminalC1. That is, by the above first operation, the AC voltage V_(p) appliedbetween the power supply points A1 and A2 is rectified and then DCvoltage is applied between the output terminals C1 and C2. The DCvoltage applied between the output terminals C1 and C2 is supplied tothe load 105.

In the above first operation, the switching of ON and OFF of theswitches 110 and 111 can be determined in the control circuit 104 inaccordance with the cycle of the AC voltage outputted from the AC powersource 203 of the power supply device 200.

FIG. 4A schematically illustrates flow of power in the wireless powersupply system in the case where the above first operation is performed.Note that in FIG. 4A, the switches 110 and 111 in the rectifier circuit103 are illustrated as a single pole double throw switch. As shown inFIG. 4A, when the rectifier circuit 103 performs the first operation,power wirelessly transmitted from the power supply device 200 issupplied to the load 105.

Note that the power receiving device 100 may have a mechanism formonitoring voltage outputted from the rectifier circuit 103 in order toconfirm whether or not the AC voltage is rectified in the rectifiercircuit 103 in the above first operation. In this case, for example, thepower receiving device 100 may be provided with an analog-to-digitalconverter for converting a voltage value outputted from the rectifiercircuit 103 from analog to digital. By a comparison of the measuredvoltage value that is digitized by the analog-to-digital converter and areference voltage value in the control circuit 104, whether or not theoperations of the switches 110 and 111 are synchronized with the cycleof the AC voltage applied from the power supply device 200 can bedetermined. When it is determined that the operations of the switches110 and 111 are not synchronized with the cycle of the AC voltage, theswitching of ON and OFF of the switches 110 and 111 may be adjusted inthe control circuit 104 such that the operations of the switches 110 and111 are synchronized with the cycle of the AC voltage.

Further, in one embodiment of the present invention, the length of theperiod during which one of the switches 110 and 111 is ON is adjusted inthe rectifier circuit 103, whereby the magnitude of the voltageoutputted from the rectifier circuit 103 can be controlled. For example,the magnitude of the voltage may be controlled in accordance with changein impedance of the load 105.

Next, in the case where the second operation is performed in therectifier circuit 103 of the power receiving device 100, the switches110 and 111 operate in accordance with a timing chart in FIG. 3B. InFIG. 3B, as in FIG. 3A, a potential difference between the power supplypoints A1 and A2 when the potential of the power supply point A2 isregarded as a reference potential in the power receiving antenna 102 isshown as voltage V_(p).

According to the timing chart in FIG. 3B, the switches 110 and 111 arekept ON. Therefore, since the power supply points A1 and A2 areshort-circuited by the second operation, the voltage V_(p) issubstantially equal to zero. Supply of power to a circuit element or awiring that is connected to the power receiving antenna 102,specifically, the capacitor 112 in the rectifier circuit 103, thecircuit elements forming the load 105, the wiring provided in therectifier circuit 103 or the load 105, and the like is stopped.Therefore, the resonant antenna 101 of the power receiving device 100does not substantially receive power from the resonant antenna 201 ofthe power supply device 200.

FIG. 4B schematically illustrates flow of power in the wireless powersupply system in the case where the above second operation is performed.Note that FIG. 4B illustrates a state where the pair of power supplypoints of the antenna element 109 of the power receiving antenna 102 inthe rectifier circuit 103 are connected using a wiring. As shown in FIG.4B, in the case where the rectifier circuit 103 performs the secondoperation, resonant coupling is not substantially formed between theresonant antenna 101 of the power receiving device 100 and the resonantantenna 201 of the power supply device 200. Accordingly, powerwirelessly transmitted from the power supply device 200 is not suppliedto the load 105.

Therefore, in one embodiment of the present invention, in the powerreceiving device 100, supply of power from the power supply device 200can be stopped without a short-circuit between the pair of power supplypoints of the antenna element 107 of the resonant antenna 101. Further,the stop of the supply of power from the power supply device 200 to thepower receiving device 100 in which charging is completed can improvepower transmission efficiency from the power supply device 200 toanother power receiving device.

Embodiment 2

In this embodiment, the structure of the power receiving device 100which uses a power storage device as the load 105 is described.

FIG. 5 illustrates the structure of the power receiving device 100according to one embodiment of the present invention. The powerreceiving device 100 in FIG. 5 is different from the power receivingdevices 100 in FIG. 1 and FIG. 2 in the structure of the load 105. InFIG. 5, the load 105 includes a power storage device 114 and a chargecontrol circuit 115. Note that the load 105 may include a load inaddition to the power storage device 114 and the charge control circuit115.

Current is supplied to the power storage device 114 using voltagetransmitted from the rectifier circuit 103, whereby charge is stored inthe power storage device 114; therefore, power is stored in the powerstorage device 114. The power storage device 114 includes at least apair of input terminals; charge is supplied from one input terminal anda reference potential such as a ground potential is applied to the otherinput terminal. When charging is completed and charge is sufficientlystored in the power storage device 114, that is, when the power storagedevice 114 is fully charged, voltage between the input terminals reachesa predetermined value Vos.

Note that it can be assumed that determination whether the power storagedevice 114 is fully charged or not depends on the practitioner. Thevalue of voltage Vos may be set as appropriate by the practitioner.

A secondary battery, a capacitor, or the like can be used as the powerstorage device 114. As the secondary battery, for example, a lead-acidbattery, a nickel-cadmium battery, a nickel-hydride battery, alithium-ion battery, or the like can be used. Further, the capacitor canbe, for example, an electric double layer capacitor, or a hybridcapacitor in which one of a pair of electrodes has an electric doublelayer structure and the other utilizes an oxidation-reduction reaction.The hybrid capacitor, for example, includes a lithium ion capacitor inwhich a positive electrode has an electric double layer structure, and anegative electrode has a lithium ion secondary battery structure.

The charge control circuit 115 has a function of preventing the powerstorage device 114 from being charged after the power storage device 114is fully charged, that is, from being overcharged. Specifically, whenthe voltage between the pair of input terminals reaches thepredetermined value Vos, the charge control circuit 115 determines thatthe power storage device 114 is fully charged, and stops supply ofcurrent to the power storage device 114.

Further, in one embodiment of the present invention, the rectifiercircuit 103 can be switched from the first operation to the secondoperation using information that the power storage device 114 is fullycharged. Specifically, the charge control circuit 115 notifies thecontrol circuit 104 of the information that the power storage device 114is fully charged. In the control circuit 104, when the above informationis notified, a signal for controlling whether each of the switches 110and 111 is ON or OFF is generated so that the rectifier circuit 103 isswitched from the first operation to the second operation. When thesecond operation is performed in the rectifier circuit 103, the resonantantenna 101 of the power receiving device 100 does not substantiallyreceive power from the resonant antenna 201 of the power supply device200.

Therefore, in one embodiment of the present invention, in the powerreceiving device 100, supply of power from the power supply device 200can be stopped without a short-circuit between the pair of power supplypoints of the antenna element 107 of the resonant antenna 101.Therefore, in the power receiving device 100 in which charging iscompleted and supply of power is unnecessary, a circuit element, awiring, and the like that are connected to the power receiving antenna102 are charged and discharged, so that power consumption can beprevented. The stop of the supply of power from the power supply device200 to the power receiving device 100 in which charging is completed canimprove transmission efficiency of power supplied to another powerreceiving device while charging.

Note that the load 105 may include a power converter circuit forconverting power outputted from the rectifier circuit 103 into powerhaving a voltage or a current that is suitable for charging of the powerstorage device 114. As the power converter circuit, a DC-DC converter orthe like can be used.

Further, in the power receiving device 100 in FIG. 5, as in the wirelesspower supply system in FIG. 1, the receiving circuit 106 may receive asignal including, as data, the cycle of the AC voltage applied from thetransmitting circuit 205 through a group of antennas for supplyingpower. Alternatively, in the power receiving device 100 in FIG. 5, as inthe wireless power supply system in FIG. 2, the receiving circuit 106may receive a signal including, as data, the cycle of the AC voltageapplied from the transmitting circuit 205 through a group of antennasdifferent from the group of antennas for supplying power. Furtheralternatively, some of the antennas may belong to both of the group ofantennas for supplying power and the group of antennas for transmittingand receiving a signal.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 3

The present inventor measured loss of power transfer in wireless powersupply from a power supply device to a power receiving device. In thisembodiment, the results are described.

The loss of power transfer was measured using different threeconditions. Under a first condition, as shown in FIG. 6A, a power supplydevice 301 and a power receiving device 303 were used. In the powerreceiving device 303, it is estimated that the first operation wasperformed in a rectifier circuit. Under the first condition, the powerreceiving device 303 was arranged within a region where a radio waveoutputted from the power supply device 301 could be received.

Under a second condition, as shown in FIG. 6B, the power supply device301, the power receiving device 303, and a power receiving device 304were used. In the power receiving devices 303 and 304, it is estimatedthat the first operation was performed in the rectifier circuit. Underthe second condition, the power receiving devices 303 and 304 werearranged within a region where a radio wave outputted from the powersupply device 301 could be received.

Under a third condition, as shown in FIG. 6C, the power supply device301 and the power receiving devices 303 and 304 were used. In the powerreceiving device 303, the first operation was performed in the rectifiercircuit. In the power receiving device 304, the second operation isperformed in the rectifier circuit. Under the third condition, the powerreceiving devices 303 and 304 were arranged within a region where aradio wave outputted from the power supply device 301 could be received.

Further, under the first to third conditions, frequency f of AC voltageoutputted from an AC power supply 302 of the power supply device 301 waschanged from 11.56 MHz to 15.56 MHz; the loss of power transfer inwireless power supply from the power supply device 301 to the powerreceiving device 303 was measured.

FIG. 7 shows measurement values of the loss of power transfer (dB) withrespect to the frequency f (MHz). In FIG. 7, a solid line denoted byPattern 1 shows a relation between the frequency f (MHz) and the loss ofpower transfer (dB) under the first condition. A solid line denoted byPattern 2 shows a relation between the frequency f (MHz) and the loss ofpower transfer (dB) under the second condition. A solid line denoted byPattern 3 shows a relation between the frequency f (MHz) and the loss ofpower transfer (dB) under the third condition.

In FIG. 7, with the frequency f of 13.56 MHz, the loss of power transferunder the first condition (the solid line denoted by Pattern 1) issubstantially equal to the loss of power transfer under the thirdcondition (the solid line denoted by Pattern 3). Accordingly, it wasproved that power transmission efficiency from the power supply device301 to the power receiving device 303 is not largely changed dependingon whether or not the power receiving device 304 in which the secondoperation is performed in the rectifier circuit exists within a regionwhere a radio wave outputted from the power supply device 301 can bereceived.

Further, in FIG. 7, with the frequency f of 13.56 MHz, the loss of powertransfer under the second condition (the solid line denoted by Pattern2) is less than that under the other conditions. Accordingly, it wasproved that the power transmission efficiency from the power supplydevice 301 to the power receiving device 303 is largely reduced when thepower receiving device 304 in which the first operation is performed inthe rectifier circuit exists within a region where a radio waveoutputted from the power supply device 301 can be received.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, more specific structures of the power receivingdevice 100 and the power supply device 200 according to one embodimentof the present invention is described.

FIG. 8 illustrates an example of a specific structure of the powerreceiving device 100. The power receiving device 100 includes theresonant antenna 101, the power receiving antenna 102, the rectifiercircuit 103, the control circuit 104, the load 105, and the receivingcircuit 106.

The resonant antenna 101 is shown in an equivalent circuit in which theantenna element 107 and the capacitor 108 are connected to each other.

The power supply points A1 and A2 of the power receiving antenna 102 areconnected to the input terminals B1 and B2 of the rectifier circuit 103,respectively. Accordingly, the potential of the power supply point A1 isapplied to the input terminal B1 and the potential of the power supplypoint A2 is applied to the input terminal B2.

Further, the rectifier circuit 103 includes a transistor 110 t servingas the switch 110 and a transistor 111 t serving as the switch 111.Furthermore, the rectifier circuit 103 includes the capacitor 112 inFIG. 8.

Note that although the case where one transistor 110 t is used as theswitch 110 is described as an example in FIG. 8, a plurality oftransistors or a circuit element other than a transistor may be used asthe switch 110. Further, although the case where one transistor 111 t isused as the switch 111 is described as an example in FIG. 8, a pluralityof transistors or a circuit element other than a transistor may be usedas the switch 111.

One of a source terminal and a drain terminal of the transistor 110 t isconnected to the input terminal B1 and the other is connected to theoutput terminal C1. One of a source terminal and a drain terminal of thetransistor 111 t is connected to the input terminal B2 and the other isconnected to the output terminal C1.

Note that a “source terminal” of a transistor means a source region thatis part of an active layer or a source electrode that is connected to anactive layer. Similarly, “drain terminal” of a transistor means a drainregion that is a part of an active layer or a drain electrode connectedto an active layer.

The control circuit 104 supplies a potential to a gate electrode of thetransistor 110 t and supplies a potential to a gate electrode of thetransistor 111 t. In this manner, the rectifier circuit 103 can performeither the first operation or the second operation in accordance withthe potentials applied from the control circuit 104 to the gateelectrode of the transistor 110 t and the gate electrode of thetransistor 111 t.

One of electrodes of the capacitor 112 is connected to the outputterminal C1 and the other of the electrodes of the capacitor 112 isconnected to the output terminal C2. Further, a reference potential suchas a ground potential is applied to the output terminal C2 and apotential difference between the output terminals C1 and C2 is smoothedby the capacitor 112. Accordingly, the smoothed potential differencebetween the output terminals C1 and C2 is applied to the load 105 as ACvoltage.

Next, FIG. 9 illustrates an example of a specific structure of the powersupply device 200. The power supply device 200 includes the resonantantenna 201, the antenna 202, the AC power source 203, the controlcircuit 204, and the transmitting circuit 205.

The resonant antenna 201 is shown in an equivalent circuit in which theantenna element 206 and the capacitor 207 are connected to each other.The antenna 202 is shown in an equivalent circuit in which the antennaelement 208 and the capacitor 209 are connected to each other.

The AC power source 203 includes a transistor 211 t, a transistor 212 t,a transistor 213 t, and a transistor 214 t serving as switches and a DCpower source 215. One of a source terminal and a drain terminal of thetransistor 211 t is supplied with voltage from the DC power source 215and the other is connected to an output terminal D2 of the AC powersource 203. A potential from the DC power source 215 is higher than areference potential such as a ground potential. One of a source terminaland a drain terminal of the transistor 212 t is connected to an outputterminal D1 of the AC power source 203 and the other is supplied with areference potential such as a ground potential. One of a source terminaland a drain terminal of the transistor 213 t is connected to the outputterminal D2 of the AC power source 203 and the other is supplied with areference potential such as a ground potential. One of a source terminaland a drain terminal of the transistor 214 t is supplied with apotential from the DC power source 215 and the other is connected to theoutput terminal D1 of the AC power source 203.

The control circuit 204 supplies a potential to a gate electrode of eachof the transistors 211 t to 214 t. The transistors 211 t to 214 t are ONor OFF, whereby the potential from the DC power source 215 and thereference potential are alternately applied to the output terminals D1and D2 and AC voltage is applied between the output terminals D1 and D2.After that, the AC voltage is supplied to the antenna 202.

The control circuit 204 controls the potentials applied to the abovegate electrodes, whereby the cycle of the AC voltage supplied from theAC power source 203 to the antenna 202 is controlled.

Next, an operation example of the power receiving device 100 in FIG. 8and the power supply device 200 in FIG. 9 is described using timingcharts in FIGS. 10A and 10B. Note that an example in which n-channeltransistors are used for all of the transistors 110 t and 111 t and thetransistors 211 t to 214 t is described in FIGS. 10A and 10B.

In the case where the first operation is performed in the rectifiercircuit 103 of the power receiving device 100, the transistors 110 t and111 t and the transistors 211 t to 214 t operate in accordance with thetiming chart in FIG. 10A. In FIG. 10A, a potential difference betweenthe power supply points A1 and A2 when the potential of the power supplypoint A2 is regarded as a reference potential in the power receivingantenna 102 is shown as voltage V_(p).

According to the timing chart in FIG. 10A, when the potential applied tothe gate electrode of each of the transistors 211 t and 212 t is ahigh-level potential, the potential applied to the gate electrode ofeach of the transistors 213 t and 214 t is a low-level potential. Thatis, when the transistors 211 t and 212 t are ON, the transistors 213 tand 214 t are OFF.

By the above operation, the reference potential is applied to the outputterminal D1 and the potential from the DC power source 215 is applied tothe output terminal D2. Therefore, when the potential of the outputterminal D2 is regarded as a reference potential, the voltage betweenthe output terminals D1 and D2 is low-level voltage. Further, thevoltage between the output terminals D1 and D2 is applied to the powersupply points A1 and A2 through the antenna 202, the resonant antenna201, the resonant antenna 101, and the power receiving antenna 102;therefore, the voltage V_(p) is low-level voltage.

Further, according to the timing chart in FIG. 10A, when the potentialapplied to the gate electrode of each of the transistors 211 t and 212 tis a low-level potential, the potential applied to the gate electrode ofeach of the transistors 213 t and 214 t is a high-level potential. Thatis, when the transistors 211 t and 212 t are OFF, the transistors 213 tand 214 t are ON.

By the above operation, the potential from the DC power source 215 isapplied to the output terminal D1 and the reference potential is appliedto the output terminal D2. Therefore, when the potential of the outputterminal D2 is regarded as a reference potential, the voltage betweenthe output terminals D1 and D2 is high-level voltage. Further, thevoltage between the output terminals D1 and D2 is applied to the powersupply points A1 and A2 through the antenna 202, the resonant antenna201, the resonant antenna 101, and the power receiving antenna 102;therefore, the voltage V_(p) is high-level voltage.

Further, according to the timing chart in FIG. 10A, when the voltageV_(p) is low-level voltage, that is, when the potential of the powersupply point A2 is higher than that of the power supply point A1, thepotential applied to the gate electrode of the transistor 110 t is alow-level potential and the potential applied to the gate electrode ofthe transistor 111 t is a high-level potential. That is, the transistor110 t is OFF and the transistor 111 t is ON. Accordingly, the potentialof the power supply point A2, which is higher than that of the powersupply point A1, is applied to the output terminal C1 through thetransistor 111 t.

Further, according to the timing chart in FIG. 10A, when the voltageV_(p) is high-level voltage, that is, when the potential of the powersupply point A1 is higher than the potential of the power supply pointA2, the potential applied to the gate electrode of the transistor 110 tis a high-level potential and the potential applied to the gateelectrode of the transistor 111 t is a low-level potential. That is, thetransistor 110 t is ON and the transistor 111 t is OFF. Accordingly, thepotential of the power supply point A1, which is higher than that of thepower supply point A2, is applied to the output terminal C1 through thetransistor 110 t.

By the above first operation, a potential that is higher than thepotential of the output terminal C2 is applied to the output terminalC1. That is, by the above first operation, the AC voltage V_(p) appliedbetween the power supply points A1 and A2 is rectified and then DCvoltage is applied between the output terminals C1 and C2. The DCvoltage applied between the output terminals C1 and C2 is supplied tothe load 105.

In the above first operation, the switching of ON and OFF of thetransistors 110 t and 111 t can be determined in the control circuit 104in accordance with the cycle of the AC voltage outputted from the ACpower source 203 of the power supply device 200.

Further, in the case where the second operation is performed in therectifier circuit 103 of the power receiving device 100, the transistors110 t and 111 t and the transistors 211 t to 214 t operate in accordancewith the timing chart in FIG. 10B. Also in FIG. 10B, a potentialdifference between the power supply points A1 and A2 when the potentialof the power supply point A2 is regarded as a reference potential in thepower receiving antenna 102 is shown as voltage V_(p).

The operations of the transistors 211 t to 214 t of the power supplydevice 200 in the case where the first operation is performed are thesame as those in the case where the second operation is performed.Therefore, when the transistors 211 t to 214 t operate in accordancewith the timing chart in FIG. 10B, low-level voltage and high-levelvoltage are alternately applied between the output terminals D1 and D2.

Further, according to the timing chart in FIG. 10B, the potentialapplied to the gate electrode of each of the transistors 110 t and 111 tremains to be high-level. That is, the transistors 110 t and 111 tremain to be ON. Therefore, the power supply points A1 and A2 areshort-circuited and thus the voltage V_(p) is substantially equal tozero. Even when low-level voltage and high-level voltage are alternatelyapplied between the output terminals D1 and D2, supply of power to acircuit element or a wiring that is connected to the power receivingantenna 102, specifically, the capacitor 112 in the rectifier circuit103, the circuit elements forming the load 105, the wiring provided inthe rectifier circuit 103 or the load 105, and the like is stopped.Therefore, the resonant antenna 101 of the power receiving device 100does not substantially receive power from the resonant antenna 201 ofthe power supply device 200.

Therefore, in one embodiment of the present invention, in the powerreceiving device 100, supply of power from the power supply device 200can be stopped without a short-circuit between the pair of power supplypoints of the antenna element 107 of the resonant antenna 101. Further,the stop of the supply of power from the power supply device 200 to thepower receiving device 100 in which charging is completed can improvepower transmission efficiency from the power supply device 200 toanother power receiving device.

Note that, in the power receiving device 100 in FIG. 8 and the powersupply device 200 in FIG. 9, as in the wireless power supply system inFIG. 1, the receiving circuit 106 may receive a signal including, asdata, the cycle of the AC voltage applied from the transmitting circuit205 through a group of antennas for supplying power. Alternatively, inthe power receiving device 100 in FIG. 8 and the power supply device 200in FIG. 9, as in the wireless power supply system in FIG. 2, thereceiving circuit 106 may receive a signal including, as data, the cycleof the AC voltage applied from the transmitting circuit 205 through agroup of antennas different from the group of antennas for supplyingpower. Further alternatively, part of the antennas may belong to both ofthe group of antennas for supplying power and the group of antennas fortransmitting and receiving a signal.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 5

A power receiving device according to one embodiment of the presentinvention is an electronic apparatus that can wirelessly receiveexternal power. Specific examples of the power receiving deviceaccording to one embodiment of the present invention include displaydevices, laptops, image reproducing devices provided with recordingmedia (typically, devices which reproduce the content of recording mediasuch as digital versatile discs (DVDs) and have displays for displayingreproduced images), cellular phones, portable game machines, personaldigital assistants, e-book readers, cameras such as video cameras anddigital still cameras, goggle-type displays (head mounted displays),navigation systems, audio reproducing devices (e.g., car audio systemsand digital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), vending machines, and thelike.

A power receiving device according to one embodiment of the presentinvention may be a moving object powered by an electric motor. Themoving object is a motor vehicle (a motorcycle or an ordinary motorvehicle with three or more wheels), a motor-assisted bicycle includingan electric bicycle, an airplane, a vessel, a rail car, or the like.

The case where power is wirelessly supplied from a power supply deviceto a plurality of moving objects that can wirelessly receive externalpower is described in this embodiment.

First, as shown in FIG. 11A, power is supplied from a power supplydevice 500 to an ordinary motor vehicle 501, an ordinary motor vehicle502, and an electric wheelchair 503 by electromagnetic resonant wirelesspower supply. The ordinary motor vehicle 501, the ordinary motor vehicle502, and the electric wheelchair 503 each include a power storage deviceand a charge control circuit as a load. Part of power supplied from thepower supply device 500 is stored in the power storage device in each ofthe ordinary motor vehicle 501, the ordinary motor vehicle 502, and theelectric wheelchair 503.

As shown in FIG. 11A, while the wireless power supply is performed, therectifier circuit in each of the ordinary motor vehicle 501, theordinary motor vehicle 502, and the electric wheelchair 503 performs thefirst operation.

After that, for example, when the power storage device in the electricwheelchair 503 is fully charged, the operation of the rectifier circuitin the electric wheelchair 503 is switched from the first operation tothe second operation in response to a signal from the charge controlcircuit. Then, the stop of the supply of power from the power supplydevice 500 to the electric wheelchair 503 in which charging is completedcan improve power transmission efficiency from the power supply device500 to the ordinary motor vehicle 501 and the ordinary motor vehicle 502(see FIG. 11B).

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

This application is based on Japanese Patent Application serial no.2011-282434 filed with Japan Patent Office on Dec. 23, 2011, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A power receiving device comprising: a firstantenna configured to form resonant coupling with an antenna of a powersupply device and receive first power from the power supply devicethrough the resonant coupling; a second antenna configured to formelectromagnetic induction coupling with the first antenna and receivesecond power from the first antenna through the electromagneticinduction coupling with the first antenna; a rectifier circuitconfigured to generate DC voltage by rectifying AC voltage applied fromthe power supply device through the first antenna and the secondantenna; a load to which the DC voltage outputted from the rectifiercircuit is applied; and a control circuit configured to generate asignal for controlling whether the DC voltage is generated by therectifier circuit or not.
 2. The power receiving device according toclaim 1, further comprising a receiving circuit, wherein the receivingcircuit extracts a signal including a cycle from AC voltage received bythe second antenna as data, and supply the signal to the controlcircuit.
 3. A power receiving device comprising: a first antennaconfigured to form resonant coupling with an antenna of a power supplydevice and receive first power from the power supply device through theresonant coupling; a second antenna configured to form electromagneticinduction coupling with the first antenna and receive second power fromthe first antenna through the electromagnetic induction coupling withthe first antenna; a rectifier circuit comprising a first switch and asecond switch, the rectifier circuit being configured to generate DCvoltage by rectifying AC voltage applied from the power supply devicethrough the first antenna and the second antenna; a load to which the DCvoltage outputted from the rectifier circuit is applied; and a controlcircuit configured to control ON or OFF of the first switch and thesecond switch, wherein the control circuit is configured so that the DCvoltage is generated by the rectifier circuit in the case where one ofthe first switch and the second switch is ON and the other of the firstswitch and the second switch is OFF, and wherein the control circuit isconfigured so that the DC voltage is not generated by the rectifiercircuit in the case where both of the first switch and the second switchare ON.
 4. The power receiving device according to claim 3, furthercomprising a receiving circuit configured to wirelessly receive a firstsignal comprising a data of a cycle of the AC voltage from the secondantenna, wherein the control circuit is configured to generate a secondsignal for selecting ON or OFF of the first switch and the second switchin response to the first signal.
 5. The power receiving device accordingto claim 3, wherein the load comprises a power storage device configuredto be charged with the DC voltage and a charge control circuitconfigured to send notification to the control circuit when the chargingis completed in the power storage device, and wherein the controlcircuit is configured to generate a signal for controlling the rectifiercircuit so that the DC voltage is not generated by the rectifier circuitwhen the charging is completed in the power storage device.
 6. The powerreceiving device according to claim 5, wherein the power storage devicecomprises a secondary battery including any one of a lead-acid battery,a nickel-cadmium battery, a nickel-hydride battery and a lithium-ionbattery.
 7. The power receiving device according to claim 5, wherein thepower storage device is an electric double layer capacitor or a hybridcapacitor which comprises a first electrode which is capable of formingan electric double layer structure and a second electrode which iscapable of undergoing an oxidation-reduction reaction when the powerstorage device is charged.
 8. The power receiving device according toclaim 3, further comprising a receiving circuit, wherein the receivingcircuit extracts a signal including a cycle from AC voltage received bythe second antenna as data, and supply the signal to the controlcircuit.
 9. A wireless power supply system comprising: a power supplydevice; and a power receiving device, the power receiving devicecomprising: a first antenna configured to form resonant coupling with anantenna of the power supply device and receive first power from thepower supply device through the resonant coupling; a second antennaconfigured to form electromagnetic induction coupling with the firstantenna and receive second power from the first antenna through theelectromagnetic induction coupling with the first antenna; a rectifiercircuit comprising a first switch and a second switch, the rectifiercircuit being configured to generate DC voltage by rectifying AC voltageapplied from the power supply device through the first antenna and thesecond antenna; a load to which the DC voltage outputted from therectifier circuit is applied; and a control circuit configured tocontrol ON or OFF of the first switch and the second switch, wherein thecontrol circuit is configured so that the DC voltage is generated by therectifier circuit in the case where one of the first switch and thesecond switch is ON and the other of the first switch and the secondswitch is OFF, and wherein the control circuit is configured so that theDC voltage is not generated by the rectifier circuit in the case whereboth of the first switch and the second switch are ON.
 10. The wirelesspower supply system according to claim 9, further comprising a receivingcircuit configured to wirelessly receive a first signal comprising adata of a cycle of the AC voltage from the second antenna, wherein thecontrol circuit is configured to generate a second signal for selectingON or OFF of the first switch and the second switch in response to thefirst signal.
 11. The wireless power supply system according to claim 9,wherein the load comprises a power storage device configured to becharged with the DC voltage and a charge control circuit configured tosend notification to the control circuit when the charging is completedin the power storage device, and wherein the control circuit isconfigured to generate a signal controlling the rectifier circuit sothat the DC voltage is not generated by the rectifier circuit when thecharging is completed in the power storage device.
 12. The wirelesspower supply system according to claim 11, wherein the power storagedevice comprises a secondary battery including any one of a lead-acidbattery, a nickel-cadmium battery, a nickel-hydride battery and alithium-ion battery.
 13. The wireless power supply system according toclaim 11, wherein the power storage device is an electric double layercapacitor or a hybrid capacitor which comprises a first electrode whichis capable of forming an electric double layer structure and a secondelectrode which is capable of undergoing an oxidation-reduction reactionwhen the power storage device is charged.
 14. The power receiving deviceaccording to claim 9, further comprising a receiving circuit, whereinthe receiving circuit extracts a signal including a cycle from ACvoltage received by the second antenna as data, and supply the signal tothe control circuit.